1. Field of the Invention
The present invention relates to a differential holographic method for measuring and testing the changing conditions of objects or articles, in which there are taken intermediate holograms of an unflawed reference object, and from which there are employed the reconstructed object waves of the reference object in order to illuminate the test object for a holographic exposure.
2. Discussion of the Prior Art
The conventional holographic method for measuring the changes in an object, such as they occur in, for instance, acoustics, in vibration analyses and deformation measurement techniques, consists of recording several holograms of various objects or object conditions on a photographic plate. During the reconstruction of the object, the superimposition of the various, as a rule of two object images, produces an interference image in which the interference bands facilitate a qualitative and quantitative indication of changes in the object.
The application of this method to nondestructive testing is predicated on the fact that flaws in an object, or deviations of a test object from a reference object will lead to changes in the interference pattern. However, inasmuch as only such flaws can be recognized through which the interference pattern is visibly disrupted relative to the unflawed condition, in many instances easily discernible evidence is not afforded by the described graphic method.
An improvement over the above-described testing method can be achieved with a so called differential holographic method, in which there is not represented the entire interference pattern resulting from the two object states, but only the deviations of the test object from a reference object.
In this known differential method, the holograms of a reference object in its original and in its deformed state are initially produced on two separate holographic plates. These holograms, herein referred to as intermediate holograms, are then successively illuminated employing the conjugated reference beam. The reconstructed object waves produced therefrom are then used in a second step for illuminating the respective object conditions of the object under test. The reconstructed image of the holograms obtained in this manner on a second holographic plate illustrates a test object pattern which is free from interference bands in the unchanged or equally shaped areas of the reference and test objects, and shows interference line fields which indicate the deviations in form or the deformation of the test object in contrast with that of the reference object.
This reduced data reproduction facilitates a significantly improved resolution and, consequently, also the measurability of minimal changes. This method has the disadvantage, however, that subsequent to developing of the two holographic plates on which the intermediate holograms have been recorded, their repositioning must be exact in order to avoid phase deviations in the reconstructed object wave fields. Considerable difficulties will also be caused by the fact that, in general, the optical thicknesses of the two holographic plates will not completely match after developing, which can again produce uncontrollable phase deviations in the reconstructed object wave fields.